The present invention relates to the use of magnetic lifting devices, in particular the safety of their use.
Magnetic lifting devices may use either permanent magnets or electromagnets, or some combination of the two, to attach themselves to loads made of ferromagnetic materials. If the attraction between the lifting device and the load is sufficiently strong, the load may be raised from the ground and moved supported upon the magnetic lifting device. Such a method of moving loads is often much more convenient than alternative moving means, such as attaching chains around the device, since the magnetic lifting device may be applied and removed so quickly.
A particular magnetic lifting device will have a maximum weight that it is able to lift, before the weight exceeds the attractive force of the magnets. It is usual to give a xe2x80x98safe working loadxe2x80x99 for a such a lifting device of a third of the maximum weight. This measure is designed to ensure that a load of a given weight will always be securely held. The weight of a particular load may not be known, and in order to weigh the load, you have to lift it. A load near to the safe lifting weight may be unstably held, and fall from the magnetic lifting device whilst it is being held aloft. Since a large lifting device may have a safe working load of 2 tones (4400 lbs.), it will be appreciated that that such an occurrence would be very dangerous.
Even if the weight of the load is known, there may be further problems. The attractive force between the magnets and the load is proportional to the amount of flux from the magnet which flows through the load. This in turn depends upon the characteristics of the load. Ideally, the load should have a clean flat surface upon which the magnetic lifting device may be placed so that there is no gap between the working surface of the magnetic lifting device and load, since some of the flux will located in this gap, diminishing the amount of flux usefully passing through the load.
Some users of magnetic lifting device resort to expedients such as pressing or banging on loads to test whether they are securely held. Such techniques have obvious drawbacks, and many potential users of magnetic lifting devices prefer to use lifting means such as chains.
The object of the present invention is to reduce the risk that loads may unexpectedly fall from a magnetic lifting device.
According to the present invention there is provided a method of testing whether a magnetic lifting device may safely lift a particular load comprising the steps of:
placing the magnetic lifting device adjacent to the load;
spacing some or all of the magnetic elements of the magnetic lifting device such that the lifting ability of the magnetic lifting device is reduced by a predetermined amount; and
attempting to lift the load by a small safe distance.
The use of the term magnetic elements of the magnetic lifting device are intended to include the magnets themselves and any pole extensions or the like of the lifting device.
Preferably the lifting amount is reduced by the introduction of a substantially planar member between the magnetic lifting device and the load. The planar member may have a similar footprint to that of the magnetic lifting device, and may be made of stainless steel.
According to another aspect of the present invention there is provided a magnetic lifting device whose magnetic elements may be spaced such that the lifting ability of the magnetic lifting device is reduced by a predictable amount.
The introduction of a shim, or other method of reducing the lifting capability of the magnetic lifting device, means that if the magnetic lifting device can lift any load under these conditions, then without such a reduction the magnetic lifting device will be able to lift the same load with a safety factor of at least the same amount as the reduction of the lifting capability of the magnetic lifting device. For example, if a shim is inserted between the magnetic lifting device and the load which reduces the lifting capability of the lifting device by a factor of three (that is, to a third of its normal capability), and the magnetic lifting device succeeds in lifting the load, then the lifting device will lift the same load without the shim with a safety factor of three.
Thus to establishing whether it is safe to use a magnetic lifting device for a particular load comprises a simple testing procedure. Knowledge of parameters which may affect the force of attraction between the load and the magnetic lifting device on the one handxe2x80x94such as the evenness of the surface, the permeability of the material, the thickness of the material, the presence of cavities in the load, and whether the surface is covered with some amount of paint or rustxe2x80x94and the weight of the load on the other hand, becomes unnecessary.